The present invention relates to a fiber optic gyro in which light propagates, as right-handed light and left-handed light, through a single-mode optical fiber coil or loop clockwise and counterclock-wise thereof and a phase difference therebetween is detected to sense an angular rate around the axis of the optical fiber coil which is applied thereto. More particularly, the invention pertains to a fiber optic gyro which employs a depolarizer for averting the influence of a polarization variation which occurs in the optical fiber coil.
FIG. 1 shows the basic arrangement of a conventional fiber gyro. Light emitted from a light source 11 passes through an optical coupler 12, such as an optical fiber coupler, and enters a polarizer 13, by which a polarized light component of only a predetermined polarization direction is extracted. The light from the polarizer 13 is split by an optical splitter/coupler 14, such as an optical fiber coupler, into two parts, one of which is coupled, as right-handed light, into one end of a single-mode optical fiber coil or loop 16 via a depolarizer 15 and the other end of the optical fiber coil 16 via an optical phase modulator 17. The right-handed light and the left-handed light, which have propagated through the optical fiber coil 16, return to the optical splitter/coupler 14, by which they are combined and interfere with each other. The resulting interference light enters the polarizer 13, by which a polarized light component of only a predetermined polarization direction is extracted, and the light having passed through the polarizer 13 is split or branched by the optical coupler 12 and is then supplied to a photodetector 18, in which it is converted to an electric signal corresponding to its intensity. The optical phase modulator 17 is driven by a periodic function signal, for example, a sine-wave signal, from a modulation signal generator 19 and the light which passes through the optical phase modulator 17 is phase modulated. The output of the photodetector 18 is applied to a synchronous detector 21, wherein it is synchronously detected by a reference signal from the modulation signal generator 19, and the detected output is provided to an output terminal 22.
In the case where no angular rate is being applied to the optical fiber coil 16 around its axis, there is no phase difference between the right-handed light and the left-handed light having propagated through the optical fiber coil 16, and the output of the synchronous detector 21 is also zero. When an angular rate is applied to the optical fiber coil 16 around its axis, a phase difference occurs between the right-handed light and the left-handed light correspondingly and the synchronous detector 21 produces an output of a polarity and a level corresponding to the direction and magnitude of the applied angular rate. Thus, the applied angular rate can be detected.
In this way, the fiber optic gyro detects the phase difference between the right-handed light and the left-handed light, but during the propagation of light through the optical fiber coil 16 polarized components are produced which are perpendicular to each other in their direction of polarization. Since the optical fiber coil 16 is slightly birefringent, the polarized light components polarized at right angles to each other differ in the propagation velocity in the optical fiber coil 16, so that interference between one of the polarized light components of the right-handed light and the other of the polarized light components of the left-handed light, which are combined by the optical splitter/coupler 14, will make it impossible to correctly detect the phase difference between the right-handed light and the left-handed light.
To avoid this, the prior art employs the depolarizer 15, by which the two polarized light components perpendicular to each other are made equal in intensity, different in phase and noncorrelating or noninterfering with each other (i.e. unpolarized) to thereby prevent interference between the one polarized light component of the right-handed light and the other polarized light component of the left-handed light.
The depolarizer 15 is usually a Lyot depolarizer with birefringent single-mode fibers (see Bohm et al., IEEE, vol. LT-1, No. 1, March 1983, page 71, for example), which is shown in FIG. 2. The Lyot depolarizer consists of two birefringent fibers 23 and 24 with different lengths L.sub.1 and 2L.sub.1, which are spliced with their perpendicular principal axes X.sub.1, Y.sub.1 and X.sub.2, Y.sub.2 displaced 45 degrees apart at the joint. In order for the depolarizer to make every incident light unpolarized, it is necessary to satisfy the following two conditions:
(a) The two pairs of perpendicularly polarized components of the light emitted from the depolarizer bear the same intensity ratio. This condition can be fulfilled by splicing the optical fibers 23 and 24 with their principal axes displaced 45 degrees apart at the joint.
(b) No correlation (or no coherence) exists between the two pairs of perpendicularly polarized components of the light emitted from the depolarizer. This condition holds when the difference in the propagation time between the light polarized in the direction of the axis X.sub.1 and the light polarized in the direction of the axis Y.sub.1 in the optical fiber 23 of the length L.sub.1 is greater than the coherence time of light. In this instance, the optical fiber 23 is required to have the length L.sub.1 which satisfies this condition.
The condition (b) is given by the following equation: ##EQU1## where .DELTA..beta. is the difference in the propagation time per unit length between the light polarized in the Y-axis direction in the birefringent fiber (i.e. the birefringency per unit length), l.sub.c is the coherence length of light (coherence time.times.light velocity) and .lambda. is the wavelength of light.
Since the depolarizer 15 is used to convert the right-handed light into incoherent X-axis and Y-axis components and the left-handed light into incoherent X-axis and Y-axis components as described above, the X-axis component of the right-handed light and the Y-axis component of the left-handed light, which are combined by the optical splitter/coupler 14, do not interfere with each other and the Y-axis component of the right-handed light and the X-axis component of the left-handed light do non interfere either, but the X-axis components of the right-handed light and the left-handed light interfere with each other and the Y-axis components of the right-handed light and the left-handed light also interfere. Either one of the interference lights is extracted by the polarizer 13 and is then supplied to the photodetector 18, and consequently, the operation of the fiber optic gyro is free from the influence of the birefringency of the optical fiber coil 16.
In Electronics Letters, 12th, April 1984, vol. 20, No. 8, page 332, the zero-point stability of the output of the fiber optic gyro of the above-mentioned construction is expressed by the following equation: ##EQU2## where .epsilon. is an amplitude leakage coefficient of the polarizer 13, .eta. is the polarization of light which is combined by the optical splitter/coupler 14, .nu. is an improvement in the rotational angle of the optical fiber with respect to the polarizer 13 and .phi..sub.o is a phase error of the fiber optic gyro between right-handed light and left-handed light (the zero-point stability of the fiber optic gyro output).
In obtaining from Eq. (2) the zero-point stability .vertline..phi..sub.0 .vertline.=1.times.10.sup.-6 rad or so, which is necessary for a fiber optic gyro of medium precision, if .epsilon.=0.01 and .nu.=0.01 are used as typical values, then .eta.=0.014 and it is necessary to make the polarization of the output light low.
The depolarizer 15 is employed for reducing the polarization of the output light of the optical fiber coil 16, but in practice it is desired to reduce the diameter of the optical fiber coil 16. The bending or twisting of the optical fiber coil may sometimes produce therein birefringence, which causes an increase in the polarization .eta. of the output light; namely, even if the X-axis and Y-axis components of light are greatly displaced apart in phase by the depolarizer 15, the birefringence in the optical fiber coil 16 may sometimes serve to reduce the phase difference between both components, resulting in the deterioration of the zero-point stability of the fiber optic gyro.
According to Springer-Verlag, "Fiber Optic Rotation Sensor and Related Technology," 1982, pages 52-77, birefringence .DELTA..beta.'=C.multidot.(r/R) is produced in a single-mode optical fiber by its bending with a radius R, where C is 1.34.times.10.sup.-6 rad/m and r is the radius of the optical fiber.